The measurement of extremely dilute concentrations of nitric oxide in biological samples is a significant measurement in physiological procedures. It is thought to be significant as a vasodilatory produced by endothelium. It is also an antithrombogenic agent. It is thought to be important as an agent controlling neuron level activities in the brain. It is thought to be a significant mediator of myocardial or cerebral ischemia. However, all of these processes where nitric oxide is important involve levels of nitric oxide that are quite small and are very difficult to measure in such small quantities. While nitric oxide analyzers are evidenced by a device such as that shown in the Parks patent (U.S. Pat. No. 4,018,562) it is difficult to obtain data from measuring instruments currently available because the concentrations of nitric oxide of interest to pathological conditions are such small trace quantities that measurement is difficult. Presently, there are no instruments available which necessarily provide measurements indicative of nitric oxide concentration in solution. In effect, this requires measurements at levels less than one part per billion.
The present disclosure sets forth a method of obtaining such measurements in tissue, for instance, or in aqueous solution where the nitric oxide (NO sometimes hereafter) may be present. The NO is, if present, available in such small quantities that it is normally obscured by the base line noise of the measuring systems. Even the finest of measuring system has a lower limit of sensitivity. The present disclosure sets forth a method and procedure for enhancing the sensitivity. Beyond that, it also describes a method of testing pathological fluids so that aqueous solutions of NO can be tested and the amount of NO can be measured. It is believed to be able to measure down to the range of about 1.0 nanomolar concentrations of NO in pathological fluids.
The present method contemplates the testing of a sample of some pathological fluid typically an aqueous solution which may or may not have NO in it. The solution is introduced into a container and is stirred continuously at a fixed rate. In the container, there is a tubing formed of a particular type of material. The preferred material is hydrophobic and permeable to NO. Typical permeable materials are made of propylene or perhaps fluorocarbons. Some are offered under the trademark Celgard (a trademark of Hoechst-Celanese Corp.) and others various specific Teflon models (a trademark of the DuPont firm). In addition, the DuPont product known as Nafion is quite successful in this regard. A container receives and holds the sample as it is stirred over an interval. A portion of the gases in the aqueous solution in the container will be absorbed into the wall of the tubing. The tubing is periodically swept by a carrier gas. The accumulated trace amount of nitric oxide and perhaps other trace gases will be absorbed into the wall of the tubing, and when the gas stream flows, the carrier gas will pick up the accumulated gases from the wall and carry them through the tubing. By controlling the scale factors namely the interval during which stirring occurs without carrier gas flow and then switching on the carrier gas, and by further controlling the temperature of the solution, the extent of stirring, the carrier gas flow rate and other scale factors, the NO is concentrated so that there is a peak at an NO detector downstream of the tubing whereby the peak is read and is proportional to the sample concentration. By fixing the scale factors, variations in concentration can be obtained which provide peaks substantially above the noise level at the threshold operation of the detector.
The foregoing describes some of the problems that relate to detection of nitric oxide. It is possible also to form other oxides of nitrogen. For instance, nitrogen forms different combinations with oxygen and it is possible therefore to form NO.sub.x. Sometimes, nitrogen will also form preferentially NO.sub.2 but this is not the only form of NO.sub.x which can be formed. Another form of nitrogen is nitrosodioxyl radical (ONOO). This radical is not the only nitrogen based radical which may be formed alternative to various oxides of nitrogen. Accordingly, these form what might be known as oxides of nitrogen generally, and they are all collectively hydrophobic in reaction with the hydrophobic membrane of the present disclosure. In a process which forms oxides of nitrogen, it is not uncommon to also form or to have available traces of oxides of sulfur which again take the form of SO.sub.x. These and other hydrophobic gases can be detected through the use of an appropriate detector. They have a similar reaction to the present reaction described for oxides of nitrogen generally speaking, and in particular nitric oxide which is probably the most common of the oxides of nitrogen. Accordingly, hydrophobic gases in general includes oxides of sulfur which also cooperate with the porous membrane described in the present disclosure.